How many nucleic acids are there

There is only one type of DNA. There are mutliple types of RNA: Messenger RNA is a temporary molecule that transports the information necessary to make a protein from the nucleus where the DNA remains to the cytoplasm, where the ribosomes are. Even though the RNA is single stranded, most RNA types show extensive intramolecular base pairing between complementary sequences, creating a predictable three-dimensional structure essential for their function.

This is known as the Central Dogma of Life, which holds true for all organisms; however, exceptions to the rule occur in connection with viral infections. Nucleic acids are molecules made up of nucleotides that direct cellular activities such as cell division and protein synthesis. Each nucleotide is made up of a pentose sugar, a nitrogenous base, and a phosphate group.

DNA carries the genetic blueprint of the cell and is passed on from parents to offspring in the form of chromosomes. It has a double-helical structure with the two strands running in opposite directions, connected by hydrogen bonds, and complementary to each other.

RNA is single-stranded and is made of a pentose sugar ribose , a nitrogenous base, and a phosphate group. RNA is involved in protein synthesis and its regulation. Answer the question s below to see how well you understand the topics covered in the previous section.

This short quiz does not count toward your grade in the class, and you can retake it an unlimited number of times. Use this quiz to check your understanding and decide whether to 1 study the previous section further or 2 move on to the next section. Privacy Policy. The primary structure of a purine is two carbon-nitrogen rings.

Each of these basic carbon-nitrogen rings has different functional groups attached to it. In molecular biology shorthand, the nitrogenous bases are simply known by their symbols A, T, G, C, and U. The difference between the sugars is the presence of the hydroxyl group on the second carbon of the ribose and hydrogen on the second carbon of the deoxyribose.

The phosphodiester linkage is not formed by simple dehydration reaction like the other linkages connecting monomers in macromolecules: its formation involves the removal of two phosphate groups. A polynucleotide may have thousands of such phosphodiester linkages. The sugar and phosphate lie on the outside of the helix, forming the backbone of the DNA.

The nitrogenous bases are stacked in the interior, like the steps of a staircase, in pairs; the pairs are bound to each other by hydrogen bonds. Every base pair in the double helivx is separated from the next base pair by 0. This is referred to as antiparallel orientation and is important to DNA replication and in many nucleic acid interactions.

Only certain types of base pairing are allowed. For example, a certain purine can only pair with a certain pyrimidine.

This is known as the base complementary rule. In other words, the DNA strands are complementary to each other. A mutation occurs, and cytosine is replaced with adenine. What impact do you think this will have on the DNA structure? RNA is usually single-stranded and is made of ribonucleotides that are linked by phosphodiester bonds. A ribonucleotide in the RNA chain contains ribose the pentose sugar , one of the four nitrogenous bases A, U, G, and C , and the phosphate group.

The mRNA is read in sets of three bases known as codons. Each codon codes for a single amino acid. In this way, the mRNA is read and the protein product is made.

Crick and Brenner showed that proflavine-mutated bacteriophages viruses that infect bacteria with single-base insertion or deletion mutations did not produce functional copies of the protein encoded by the mutated gene. The production of defective proteins under these circumstances can be attributed to misdirected translation. Mutant proteins with two- or four-nucleotide insertions or deletions were also nonfunctional.

However, some mutant strains became functional again when they accumulated a total of three extra nucleotides or when they were missing three nucleotides.

This rescue effect provided compelling evidence that the genetic code for one amino acid is indeed a three-base, or triplet, code. However, at the time when this decoding project was conducted, researchers did not yet have the benefit of modern sequencing techniques. To circumvent this challenge, Marshall W. Nirenberg and Heinrich J. Matthaei made their own simple, artificial mRNA and identified the polypeptide product that was encoded by it. To do this, they used the enzyme polynucleotide phosphorylase, which randomly joins together any RNA nucleotides that it finds.

Nirenberg and Matthaei began with the simplest codes possible. Specifically, they added polynucleotide phosphorylase to a solution of pure uracil U , such that the enzyme would generate RNA molecules consisting entirely of a sequence of U's; these molecules were known as poly U RNAs.

These poly U RNAs were added to 20 tubes containing components for protein synthesis ribosomes , activating enzymes, tRNAs, and other factors. Each tube contained one of the 20 amino acids, which were radioactively labeled. Of the 20 tubes, 19 failed to yield a radioactive polypeptide product. Only one tube, the one that had been loaded with the labeled amino acid phenylalanine, yielded a product.

Nirenberg and Matthaei had therefore found that the UUU codon could be translated into the amino acid phenylalanine. These eight random poly AC RNAs produced proteins containing only six amino acids: asparagine, glutamine, histidine, lysine, proline, and threonine.

With the random sequence approach, the decoding endeavor was almost completed, but some work remained to be done. Thus, in , H. Gobind Khorana and his colleagues used another method to further crack the genetic code. These researchers had the insight to employ chemically synthesized RNA molecules of known repeating sequences rather than random sequences. They showed that a short mRNA sequence—even a single codon three bases —could still bind to a ribosome , even if this short sequence was incapable of directing protein synthesis.

The ribosome-bound codon could then base pair with a particular tRNA that carried the amino acid specified by the codon Figure 2. Nirenberg and Leder thus synthesized many short mRNAs with known codons.

They then added the mRNAs one by one to a mix of ribosomes and aminoacyl-tRNAs with one amino acid radioactively labeled. For each, they determined whether the aminoacyl-tRNA was bound to the short mRNA-like sequence and ribosome the rest passed through the filter , providing conclusive demonstrations of the particular aminoacyl-tRNA that bound to each mRNA codon.

Examination of the full table of codons enables one to immediately determine whether the "extra" codons are associated with redundancy or dead-end codes Figure 3. Note that both possibilities occur in the code.

There are only a few instances in which one codon codes for one amino acid, such as the codon for tryptophan. Moreover, the genetic code also includes stop codons, which do not code for any amino acid. The stop codons serve as termination signals for translation. When a ribosome reaches a stop codon, translation stops, and the polypeptide is released.

Crick, F. General nature of the genetic code for proteins. Nature , — link to article. Jones, D. Further syntheses, in vitro, of copolypeptides containing two amino acids in alternating sequence dependent upon DNA-like polymers containing two nucleotides in alternating sequence.

Journal of Molecular Biology 16 , — Leder, P. Cell-free peptide synthesis dependent upon synthetic oligodeoxynucleotides. Proceedings of the National Academy of Sciences 50 , — Nirenberg, M. An intermediate in the biosynthesis of polyphenylalanine directed by synthetic template RNA. Proceedings of the National Academy of Sciences 48 , — Approximation of genetic code via cell-free protein synthesis directed by template RNA.

Federation Proceedings 22 , 55—61 Nishimura, S.